1. Field of the Invention
The present invention relates to ophthalmic instruments that aid in detection and diagnosis of eye disease, pre-surgery preparation and computer-assisted eye surgery (such as laser refractive surgery), including ophthalmic imaging and/or topography instruments (such as fundus cameras, corneal imaging devices, retinal imaging devices, corneal topographers, and retinal topographers) in addition to ophthalmic examination instruments (such as autorefractors, slit lamps and other indirect opthalmoscopes).
2. Summary of the Related Art
The optical system of the human eye has provided man with the basic design specification for the camera. Light comes in through the cornea, pupil and lens at the front of the eye (as the lens of the camera lets light in). This light is then focused on the inside wall of the eye called the retina (as on the film in a camera). This image is detected by detectors that are distributed over the surface of the retina and sent to the brain by the optic nerve which connects the eye to the brain (as film captures the image focused thereon).
FIG. 1 shows a horizontal cross section of the human eye. The eye is nearly a sphere with an average diameter of approximately 20 mm. Three membranes—the cornea and sclera outer cover, the choroid and the retina—enclose the eye. The cornea 3 is a tough transparent tissue that covers the anterior surface of the eye. Continuous with the cornea 3, the sclera 5 is an opaque membrane that encloses the remainder of the eye. The choroid 7 lies directly below the sclera 5 and contains a network of blood vessels that serves as the major source of nutrition to the eye. At its anterior extreme, the choroid 7 includes a ciliary body 9 and an iris diaphragm 11. The pupil of the iris diaphragm 11 contracts and expands to control the amount of light that enters the eye. Crystalline lens 13 is made up of concentric layers of fibrous cells and is suspended by fibers 15 that attach to the ciliary body 9. The crystalline lens 13 changes shape to allow the eye to focus. More specifically, when the ciliary muscle in the ciliary body 9 relaxes, the ciliary processes pull on the suspensory fibers 15, which in turn pull on the lens capsule around its equator. This causes the entire lens 13 to flatten or to become less convex, enabling the lens 13 to focus light from objects at a far away distance. Likewise, when the ciliary muscle works or contracts, tension is released on the suspensory fibers 15, and subsequently on the lens capsule, causing both lens surfaces to become more convex again and the eye to be able to refocus at a near distance. This adjustment in lens shape, to focus at various distances, is referred to as “accommodation” or the “accommodative process” and is associated with a concurrent constriction of the pupil.
The innermost membrane of the eye is the retina 17, which lies on the inside of the entire posterior portion of the eye. When the eye is properly focused, light from an object outside the eye that is incident on the cornea 3 is imaged onto the retina 17. Vision is afforded by the distribution of receptors (e.g., rods and cones) over the surface of the retina 17. The receptors (e.g., cones) located in the central portion of the retina 17, called the fovea 19 (or macula), are highly sensitive to color and enable the human brain to resolve fine details in this area. Other receptors (e.g., rods) are distributed over a much larger area and provides the human brain with a general, overall picture of the field of view. The optic disc 21 (or the optic nerve head or papilla) is the entrance of blood vessels and optic nerves from the brain to the retina 17. The inner part of the posterior portion of the eye, including the optic disc 21, fovea 19 and retina 17 and the distributing blood vessels is called the ocular fundus 23.
A fundus camera is an optical instrument that enables a practitioner to view (and typically capture) an image of the ocular fundus 23 (or portions thereof) to aid the practitioner in the detection and diagnosis of disease in human eye. The fundus camera typically includes two different illumination sources—an observation source and a photographing source. The observation source, which is typically a halogen lamp or infra-red light source, is used during an observation mode of operation to view (observe) the ocular fundus 23 (or portions thereof) typically through a view finder. The photographing source, which is typically a xenon flash lamp, is used during a photographing mode of operation to photograph on film and/or capture on a CCD camera body, an image of the ocular fundus 23 (or portion thereof).
The fundus camera includes an optical subsystem that illuminates the ocular fundus 23 and collects the light reflected therefrom to produce an image of the ocular fundus 23. In the observation mode of operation, the observation source is activated (and the photographing source is de-activated). The optical subsystem illuminates the ocular fundus 23 with light produced from the observation source and collects the light reflected therefrom to produce an image of the ocular fundus 23 (or portions thereof) for view typically through a view finder. In the photographing mode of operation, the photographing source is activated (and the observation source is de-activated). The optical subsystem illuminates the ocular fundus 23 with light produced from the photographing source and collects the light reflected therefrom to produce an image of the ocular fundus 23 (or portions thereof) for capture on film or on the CCD camera body.
In addition, as is well known in the art, the optical subsystem of the fundus camera may include narrow band spectral filters for use in the photographing mode of operation to enable fluorescein angiography and/or indocyanine green angiography.
Examples of prior art fundus cameras are described in U.S. Pat. Nos. 4,810,084; 5,557,321; 5,557,349; 5,617,156; and 5,742,374; each herein incorporated by reference in its entirety.
Current fundus cameras suffer from the problem that the aberrations of the eye limit the resolution of the camera. More specifically, defocus such as myopia (near-sightedness) or hyperopia (far-sightedness) and astigmatism as well has many other higher order aberrations not only blur images formed on the retina (thus impairing vision), but also blur images taken of the retina of the human eye. A more detailed discussion of such aberrations is described by Williams et al. in “Visual Benefit of Correcting Higher Order Aberrations of the Eye,” Journal of Refractive Surgery, Vol. 16, September/October 2000, pg. S554-S559.
In U.S. Pat. Nos. 5,777,719, 5,949,521, and 6,095,651, Williams and Liang disclose a retinal imaging method and apparatus that produces a point source on a retina by a laser. The laser light reflected from the retina forms a distorted wavefront at the pupil, which is recreated in the plane of a deformable mirror and a Schack-Hartmann wavefront sensor. The Schack-Hartmann wavefront sensor includes an array of lenslets that produce a corresponding spot pattern on a CCD camera body in response to the distorted wavefronts. Phase aberrations in the distorted wavefront are determined by measuring spot motion on the CCD camera body. A computer, operably coupled to the Schack-Hartmann wavefront sensor, generates a correction signal which is fed to the deformable mirror to compensate for the measured phase aberrations. As discussed in column 7, lines 14-37, after correction has been achieved via the wavefront sensing of the reflected retinal laser-based point source, a high-resolution image of the retina can be acquired by imaging a krypton flash lamp onto the eye's pupil and directing the reflected image of the retina to the deformable mirror, which directs the reflected image onto a second CCD camera body for capture. Examples of prior art Schack-Hartmann wavefront sensors are described in U.S. Pat. Nos. 4,399,356; 4,725,138, 4,737,621, and 5,529,765; each herein incorporated by reference in its entirety.
Notably, the retinal imaging method and apparatus of Williams and Liang, supra, utilizes two different light sources—a laser light source and a krypton flash lamp—to perform the wavefront measurement and correction operations and imaging operations. Such a design significantly increases the complexity and cost of the system.
In addition, the retinal imaging method and apparatus Williams and Liang cannot correct for aberrations (such as those due to blinking or accommodation) that occur after the wavefront sensing and compensation operations are complete (for example, during the subsequent imaging operations).
In addition, the retinal imaging method and apparatus of Williams and Liang does not permit the user to view (observe) the ocular fundus through a view finder, which limits the applications of the retinal imaging method and apparatus of Williams and Liang.
In addition, the Schack-Hartmann wavefront sensor of the retinal imaging apparatus of Williams and Liang is susceptible to the dot crossover problem. More specifically, in a highly aberrated eye, the location of spots produced on the CCD camera body may overlap (or cross). Such overlap (or crossover) introduces an ambiguity in the measurement that must be resolved, or an error will be introduced.
Other ophthalmic imaging instruments (such as corneal topographers, retinal topographers, corneal imaging devices and retinal imaging devices) suffer from these same limitations. A corneal topographers is an ophthalmic instrument that projects light (such as a series of illuminated rings or light slits) onto the anterior corneal surface, which are reflected back into the instrument. The reflections are analyzed by the instrument and a topographical map of the anterior surface of the cornea (and possibly of the posterior surface and thickness of the cornea) is generated. The topographical map and computerized analysis reveals any distortions of the cornea. Alternatively, corneal topographers may use optical coherent tomography to image and characterize the thickness of the corneal epithelium and characterize the 3-D structure of the cornea. Retinal topographers utilize similar techniques to characterize the structure of the retina. Corneal imaging devices capture high resolution images (typically utilizing confocal microscopy, such as laser confocal scanning microscopy) of the various portions of the cornea of the human eye. In addition, such corneal imaging devices may derive high resolution tomography of such corneal portions from analysis of the captured images. Retinal imaging devices utilize similar techniques to capture high resolution images of the various portions of the retina of the human eye. In addition, such retinal image devices may derive high resolution tomography of such retinal portions from analysis of the captured images.
In addition, current ophthalmic examination instruments (including retinoscopes, autorefractors, slit lamps and other indirect opthalmoscopes) do not measure and characterize the higher order aberrations of the human eye, which may be required for adequately diagnosing and treating the patient. A retinoscope (or phoropter) is an ophthalmic instrument that subjectively measures the refractive error of the eye. An autorefractor is an ophthalmic instrument that objectively measures the refractive error of the eye. The retinoscope and autorefractor characterize the refractive errors of the eye only in terms of focal power (typically measured in diopter) required to correct for such focal errors. A slit lamp is an ophthalmic instrument that includes a moveable light source and binocular microscope with which the practitioner can examine the eye. It is used by itself to evaluate the anterior segment of the eye, and when combined with special lenses, adapts for examination of the posterior segment of the eye. An indirect opthalmoscope is an ophthalmic instrument that allows the observer to gain a view of the cornea, retina or other portion of the eye. A light source from the indirect opthalmoscope is directed into the patient's eye and the reflected light is gathered by a condensing lens to form an image of the patient's eye under observation. This image is viewed by the practitioner through a view finder and/or through image capture and display.
Thus, there is a great need in the art for an improved ophthalmic instruments, including ophthalmic imaging instruments and ophthalmic examination instruments, that avoid the shortcomings and drawbacks of prior art ophthalmic instruments.